1. Field of the Invention
The present invention relates to a photoelectric conversion device using a single crystal semiconductor or a polycrystalline semiconductor and to a method for manufacturing the photoelectric conversion device.
2. Description of the Related Art
Since the situation of global warming has been serious, a measure against the global warming is an issue that needs to be addressed worldwide. Most greenhouse effect gases which are regarded as a main cause for global-warming, such as carbon dioxide, are exhausted by consumption of energy, such as oil, coals, and natural gases. However, such energy is indispensable to the industrial society. Therefore, the amount of energy consumption cannot be reduced simply. Thus, solar-electric power generation which discharges less amount of carbon dioxide and which is eco-friendly has attracted attentions as the next-generation energy.
For the solar-electric power generation, a silicon photoelectric conversion device, which converts light energy into electric energy utilizing semiconductor characteristics, is widely applied. Since silicon photoelectric conversion devices typified by solar cells are already available in the market and government around the world support solar cells, the production thereof has been expanding year by year. For example, the production of solar cells around the world in 2006 is 2521 MW, which has increased by over 40% per annum.
However, there are many obstacles to the widespread of solar-electric power generation. One of the obstacles is a higher cost of solar-electric power generation compared with a cost of conventional commercial power. In order to reduce a cost for photoelectric power generation, improvement of efficiency in a solar cell, reduction of costs such as manufacturing costs, and the like are indispensable.
Further, the present obstacle to the widespread of solar-electric power generation is short supply of silicon. The supply-demand balance of silicon had been excess in supply reflecting semiconductor recession. In contrast, silicon is now short of supply since around fiscal 2005 because of drastic growth of the solar cell market with the recovery of the semiconductor (LSI) industry. Though major silicon suppliers in the world have already tried to increase capability of silicon production, the increase in demand outweighs the capability.
While shortage of silicon materials is obvious, a thin film silicon photoelectric conversion device has attracted attentions. In the thin film silicon photoelectric conversion device, a thin silicon film is formed over a supporting substrate and the thin silicon film functions as a photoelectric conversion layer. In contrast, a bulk silicon photoelectric conversion device, which is mainly used currently, includes a main portion formed from a single crystal silicon substrate or a polycrystalline silicon substrate having a thickness which is more than several tens times or more as large as a thickness necessary for the photoelectric conversion layer. Therefore, it is hard to say that silicon is used effectively in the bulk silicon photoelectric conversion device. Speaking of extremes, most part of the single crystal silicon substrate or the polycrystalline silicon substrate which is used for the bulk silicon photoelectric conversion device functions only as a structural body for keeping the shape of the photoelectric conversion device. The thin film silicon photoelectric conversion device can have a structure in which silicon is used only for a region for converting light into electricity. Therefore, a silicon usage can be reduced drastically compared with a silicon usage of the bulk silicon photoelectric conversion device.
However, a thin film silicon photoelectric conversion device has lower photoelectric conversion efficiency as compared to a bulk silicon photoelectric conversion device because, generally in a thin film silicon photoelectric conversion device, a thin silicon film is formed over a supporting substrate by utilizing a variety of chemical or physical vapor deposition methods. Therefore, a method for manufacturing a solar cell is proposed in which hydrogen ions are implanted into a crystalline semiconductor and the crystalline semiconductor is cut by heat treatment to obtain a crystalline semiconductor layer (for example, see Patent Document 1: Japanese Published Patent Application No. H10-335683). After a crystalline semiconductor in which ions of a predetermined element are implanted in a layered manner is attached to a surface of electrode-forming paste applied over a substrate provided with an insulating layer, heat treatment is performed at 300° C. to 500° C. to bond the crystalline semiconductor to an electrode. Next, by heat treatment at 500° C. to 700° C., voids distributed in a layered manner are formed in a region of the predetermined element implanted into the crystalline semiconductor, and further, the crystalline semiconductor is divided at the voids by heat distortion. Accordingly, a crystalline semiconductor layer over the electrode is obtained. Furthermore, an amorphous silicon layer is formed over the crystalline semiconductor layer. Thus, a tandem-type solar cell is manufactured. In this method, a single crystal silicon solar battery cell to be a first power generation layer is formed.